A Session Initiation Protocol (SIP) Uniform Resource Identifier (URI) is the SIP addressing schema to call another person via SIP. In other words, a SIP URI is a user's SIP phone number. The SIP URI resembles an e-mail address and is written in the following format: SIP URI=sip:x@y:Port, where x=Username and y=host (domain or IP). The SIP URI standard has been defined in the RFC 3261 standard, the entire contents of which are hereby incorporated herein by reference.
When SIP Back to Back User Agents (B2BUAs) exist between a conference participant User Agent (UA) and a conference focus that is being operated in accordance with RFC 4579, the entire contents of which are hereby incorporated herein by reference, problems can occur. Specifically, conference participants should be able to SUBSCRIBE for conference state events and REFER other UAs to join the conference. Both of these messages use the Contact URI of the conference focus (e.g., conference server, conference bridge, mixer location, etc.) to identify the exact focus of the conference call.
B2BUAs situated between the conference focus and the UA in the signaling path, however, will have changed this Contact URI. The issue then revolves around the question of how to enable SUBSCRIBE/REFER to the conference focus when intermediate B2BUAs have changed the Contact URI of the conference focus. If the problem is not addressed, SIP UAs and conference bridges that are in complete alignment with the standards defined in RFC 3261 and RFC 4579 will simply not be able to create ad-hoc conferences or subscribe to conference state events in a B2BUA environment.